Large capacity blowers for industrial use are already well known and widespread in numerous sectors of industry. Such blowers can be applied in sectors of industry that are as varied as: the nuclear industry, the chemical industry, the iron and steel industry, the cement Industry, for example, or Indeed in fossil fuel power stations.
In these industrial applications, it is conventional to distinguish between two major type. of blower unit: centrifugal type blower units; and axial type blower units; it being understood that the present invention relates more particularly to centrifugal type blowers in which the fluid to be blown is sucked in laterally relative to the blower wheel through At least two inlets, and is then expelled by the centrifugal force created by the vanes of the blower wheel.
Given the large ventilation capacities required by the above-mentioned industrial applications, for example of the order of 500 kW to 5000 kW, and indeed in extreme cases 100 kW to 10,000 kW, blower unit designs are of large dimensions, even though such units are made up of moving parts which are subject in use to high stresses, mechanically, in terms of duration of operation, and in terms of the conditions of the medium in which the parts need to operate.
A recognized consequence is that criteria such as operational effectiveness, technical reliability, noise level, and operating cost, including the notion of blower efficiency, are of very great importance when designing and using such devices.
Proposals have thus already been made to control and vary the suction flow rate of two-inlet centrifugal blowers in order to reduce the power they absorb, for the purpose of obtaining aerodynamic control of the flow rate of the device, necessary for better efficiency and thus for guaranteed savings.
Thus, it is already known to reduce the flow rate of two-inlet centrifugal blowers in a given circuit by fitting them with air-deflector elements such as deflectors or suction dampers. A deflector is formed by a circular disposition of blades mounted at a suction inlet on one side or the other of the blower wheel to turn about respective radial axes, e.g. centered on the axis of the wheel. A suction damper is likewise formed by a set of blades mounted in a cover, with the ability to be turned or varied in angular pitch about parallel axes.
In both the above-described prior art embodiments, the deflector elements are functionally connected to at least one means for controlling their position and pitch, and to conventional drive means such as a servo-motor. It is thus possible to position the deflector elements associated with an inlet so as to vary the suction flow rate.
It is known that by tilting the deflector elements in an appropriate manner, a rotary flow of air is created at the inlet to the blower wheel, which flow is usually directed to rotate in the same direction as the blower wheel. Compared with other systems for controlling the flow in the circuit, this rotary air flow has the consequence of reducing the power absorbed by the blower. When the angular displacement of the deflector elements associated with each inlet is obtained in symmetrical manner, the suction flow rate is identical or substantially identical through each inlet, and the results obtained with such apparatuses are represented by curve c shown in FIG. 1. Depending on the pitch of the deflector elements and on the operating conditions of the blower, expressed in percentage terms as a function of the maximum rate authorized for the installation, this curve shows how efficiency varies as a function of flow rate on a given installation, with the resistance of the system varying in proportion with the square of the flow rate.
It is generally considered that such flow rate control systems give overall satisfaction since they make it possible to achieve a relative improvement in device efficiency, Nevertheless, it turns out that the efficiencies obtained are insufficient, particularly in the normal operating ranges of blower units, corresponding, for example, to operating at partial loads of the order of 40% to 80%.
To improve the efficiency of blower units operating under partial load, proposals have already been made in Published German application No. 2538066, in the case of centrifugal blowers fitted with deflector control systems, to control the orientation of the deflectors of one inlet asynchronously or asymmetrically relative to the other. This asymmetry of operation in the suction flow makes it possible to achieve a significant improvement in the efficiency of a blower unit when partially loaded. Nevertheless, it appears that the economic optimum operating range for such devices is obtained at high partial loads.
Also, the cost of such apparatuses, particularly for high power blower machines, turns out to be a factor that can limit the use thereof.